Crouch Pivoting
From Ant-d: “During a normal dash it is possible to turn around without lag by pressing down first, then the direction you're turning to.
Yeah I found this by 'accident' when making my Ness video. I wanted to immediately turn but I couldn't, so I guessed that maybe holding (really, tapping) down for 1 or 2 frames then turning would work. And it did. I tried to do a smash attack other than usmash and I don't think it worked. I'll test it now.
Edit: You can't smash. You can jab though, but that's it. I should add that the timing seems to randomly change from pressing down for 1-2 frames. I think this would be very, very hard to pull off normally. Holding down when dashing really does help you to stop. That is if you want to stop and perform an attack.